To fully understand the function of the nuclear envelope in regulating cell activity, comparative data of nuclear permeability in a variety of different cell types are necessary. For technical reasons, in vivo permeability studies have been limited to a few large cells. To overcome this deficiency, experiments are planned in which nuclei from embryonic and differentiated Xenopus cells will be transplanted into oocytes or unfertilized eggs. Under these conditions, the permeability coefficients of the envelopes can be determined by injecting labelled macromolecules into the cells, and measuring nuclear uptake using autoradiographic techniques. The distribution of oligosaccharides along the membranes of the nuclear envelope, and within the pores will be studied at the ultrasound level using ferritin-conjugated lectins. Oligosaccharides are involved in specific molecular interactions at the cell surface, and might have similar functions at the nuclear surface, especially relating to nucleocytoplasmic exhanges. Amino acid analysis will be performed on nuclear polypeptides eluted from SDS-gels in order to establish whether a relationship exists between the size and charge of molecules which enter the nucleus. Such data will be useful in evaluating the importance of molecular charge in regulating passage across the envelope.